In a gas turbine, an axial flow compressor supplies air under pressure for expansion through a turbine section and typically comprises a rotor surrounded by a casing. The casing generally comprises two half cylindrical sections, removably joined together. The rotor includes a plurality of stages, each comprising a rotor disc with a single row of blades located about its outer rim. The stages are joined together and to a turbine driven shaft. The casing supports a plurality of stages or annular rows of stator vanes. The stator vane stages are located between the compressor blade stages, helping to compress the air forced through the compressor and directing the air flow into the next stage of rotor blades at the proper angle to provide a smooth, even flow through the compressor.
It has long been known that the use of variable stators to control the amount of air flowing through the compressor will optimize the performance of the compressor throughout the entire operating range of the engine. To this end, selected stator vane stages (generally at the forward portion of the compressor) are provided with variable stator vanes. In the usual prior art practice, at the position of each variable stator vane, the casing is provided with an opening or bore surrounded by an exterior boss. The variable stator vane itself has a base and/or a shaft portion which extends through the bore and is rotatable therein. A bearing assembly is provided in association with the bore to prevent wear of the casing and the stator vane.
Through appropriate testing, a stator schedule is developed which optimizes performance of the compressor, while maintaining acceptable stall margins, throughout the range of operation of the engine. An actuation system is provided to rotate and reposition the stator vanes of each variable stator vane stage according to the stator schedule.
In the usual practice, a shiftable unison ring is provided for each variable stage and surrounds the casing. Each variable stator vane of each variable stage has a lever arm operatively connected to its respective unison ring. The unison rings are shifted by an appropriate drive or bell crank mechanism operated by an appropriate actuator, as is well known in the art.
The above-mentioned bearing assembly, designed to protect the variable stator vane and the adjacent portion of the casing, are, of course, subject to wear. This can lead to metal-to-metal contact between a variable stator vane and the compressor casing. Excessive metal-to-metal contact increases friction in the variable vane system, which in turn can prevent or interfere with movement of the vanes which could result in engine stall. The bearing assembly includes bushings which wear as the variable stator vane is pivoted during engine operation. Some portions of the bushings which are highly loaded tend to wear more than other less highly loaded portions. In prior art bearing assemblies of this type, unacceptable wear has been detected a range within about 6,000 to 10,000 hours of engine operation.
Maintenance to replace the bushings involves removing the compressor casing and tearing down the variable stator vane assembly. This is expensive, time-consuming and requires skilled workers.
More particularly in the prior art stator vane assemblies, for example, those illustrated in FIG. 1 hereof, there is typically provided a thrust washer 10 disposed in an inside diameter counterbore 11 of a compressor casing 12. A bushing 14 is also typically provided, along an outside diameter counterbore 15 of the casing 12. The stator vane 16 has a radial outer vane button 18 which is inserted into the inside diameter counterbore 11. To secure the vane, a spacer 20 overlies the vane and has a central opening through which a spindle 22 projects, terminating in an externally threaded spindle portion 24. A lever arm 26 is received over the spindle 22 and the assembly is secured by a nut 28 threaded on the spindle portion 24, clamping a sleeve 30 against lever 26 and spacer 20, and button 18 against thrust washer 10. Typically, the lever arm is connected to the unison ring 30 through a pin 32. A drive mechanism, not shown, displaces ring 30 to control the pivotal location of lever 26 and hence the angle of the stator vane in accordance with a predetermined schedule.
The radial pressure load on the vane button 18 is carried through the thrust washer 10 and is reactive at the inside diameter of the compressor casing. This radial load, together with the rotational torque of the vane, causes the washer 10 to prematurely wear. Once worn, it accelerates the wear of bushing 14, causing metal-to-metal contact between the vane and the casing. This increased wear enables the vane angle to drift from the desired design angle and causes adjacent rotor blade failure and costly and extensive damage to the compressor. However, to replace the interior washer 10, all the engine piping, compressor casing halves and the entire variable stator vane system must be disassembled, resulting in costly downtime.
This problem has been addressed in U.S. Pat. No. 5,308,226, titled "Variable Stator Vane Assembly for an Axial Flow Compressor of a Gas Turbine Engine." In that patent, a somewhat complex stator vane assemblage is disclosed. It permits the parts thereof which wear, i.e., the bushing, to be removed and replaced or the entire stator vane mounting assembly to be rotated 180.degree. from outside the casing and without removal of the casing or stator vane. In that manner, the service life of the assemblage and the compressor can be greatly extended. The assemblage disclosed in that patent, however, requires a substantial number of machined parts and a complexity of assemblage which, while effective to permit rotation or removal and replacement of the bushing, remains somewhat expensive and labor-intensive.